SI-8601 Avoid over-eager optimization of LOAD_FIELD

It can NPE or trigger static class initilization, we can't elimiate
it without changing semantics.

To make sure we don't thwart closure elimination, I've allowed DCE
to eliminate a non-static LOAD_FIELD of a member of a closure class.
It would be more general to track nullity of the reciever (e.g, `this`
or `new Foo` cannot be null), but that would require more
infrastructure in this phase.

I've added a test for closure inlining based on a a suggestion by
@dragos. This actually passes if we remove the (LOAD_FIELD, DROP)
peephole optimization for `closelim` altogether. But I chose to
adapt that optimization (only allow it for non-static, closure
fields), rather then remove it alogether, in the interests of
treading lightly.

4 files changed, 40 insertions, 0 deletions

diff --git a/test/files/run/t8601-closure-elim.flags b/test/files/run/t8601-closure-elim.flags
new file mode 100644
index 0000000000..49d036a887
--- /dev/null
+++ b/test/files/run/t8601-closure-elim.flags
@@ -0,0 +1 @@
+-optimize
diff --git a/test/files/run/t8601-closure-elim.scala b/test/files/run/t8601-closure-elim.scala
new file mode 100644
index 0000000000..2c5b03af77
--- /dev/null
+++ b/test/files/run/t8601-closure-elim.scala
@@ -0,0 +1,26 @@
+import scala.tools.partest.BytecodeTest
+import scala.tools.asm
+import scala.tools.asm.util._
+import scala.collection.JavaConverters._
+
+object Test extends BytecodeTest {
+  val nullChecks = Set(asm.Opcodes.NEW)
+
+  def show: Unit = {
+    def test(methodName: String) {
+      val classNode = loadClassNode("Foo")
+      val methodNode = getMethod(classNode, "b")
+      val ops = methodNode.instructions.iterator.asScala.map(_.getOpcode).toList
+      assert(!ops.contains(asm.Opcodes.NEW), ops)// should be allocation free if the closure is eliminiated
+    }
+    test("b")
+  }
+}
+
+class Foo {
+  @inline final def a(x: Int => Int) = x(1)
+  final def b {
+    val delta = 0
+    a(x => delta + 1)
+  }
+}
diff --git a/test/files/run/t8601c.flags b/test/files/run/t8601c.flags
new file mode 100644
index 0000000000..1182725e86
--- /dev/null
+++ b/test/files/run/t8601c.flags
@@ -0,0 +1 @@
+-optimize
\ No newline at end of file
diff --git a/test/files/run/t8601c.scala b/test/files/run/t8601c.scala
new file mode 100644
index 0000000000..c487d6825e
--- /dev/null
+++ b/test/files/run/t8601c.scala
@@ -0,0 +1,12 @@
+object Test {
+  def loadField(x: scala.runtime.IntRef): Unit = x.elem
+  def storeField(x: scala.runtime.IntRef): Unit = x.elem = 42
+
+  def check(x: => Any) = try { x; sys.error("failed to throw NPE!") } catch { case _: NullPointerException => }
+
+  def main(args: Array[String]) {
+    check(loadField(null)) // bug: did not NPE under -Ydead-code
+    check(storeField(null))
+
+  }
+}